I won't bore you with the chemistry, but the trans forms of fats are lower in energy and are thermodynamically favored when and if the fat molecules rearrange, so it is a challenge to hydrogenate oils without having them do this.

When William Procter and James Gamble started the company Procter & Gamble, they hired chemist E. C. Kayser and developed the process to hydrogenate cottonseed oil, which ensures the shortening remains solid at normal storage temperatures.